1. Field of the Invention
This specification relates to a fast switch, a component of a fault current limiter, and more particularly, a fast switch capable of constantly performing a main circuit interrupting operation, regardless of a size of a fault current, by interrupting a main circuit using a discharge current of an external capacitor.
2. Background of the Invention
Generally, a fault current limiter is a power device for protecting a power system by rapidly reducing a fault current when the large fault current occurs on the power system. That is, when a large fault current occurs on a power system, the fault current limiter reduces the fault current to a proper value or less than within a short time, thereby reducing a mechanical and thermal stress of the power device and enhancing reliability of the power system.
Such a fault current limiter may be compared with a general circuit breaker as follows. When a fault current occurs, the fault current limiter detects a breakdown rapidly and introduces a resistance (impedance). On the other hand, when a fault current occurs, the general circuit breaker separates or excludes a breakdown-occurred line from a power system by an interrupting operation. Further, it takes about 16 ms for the fault current limiter to operate after the fault current has occurred. On the other hand, it takes about 85 ms˜120 ms for the general circuit breaker to operate after the fault current has occurred. Further, the fault current limiter is provided with a circuit for reducing a mechanical and thermal stress generated due to a breakdown, and for compensating for a low voltage. On the other hand, the general circuit breaker is not provided with such functions.
In a power system, when power of high quality is required and power has a large capacity, the fault current limiter is preferred owing to such advantages.
Main components of the fault current limiter include a fast fault detector (FFD), a fast switch (FS), and a current limiting resistor (CLR).
The fast fault detector (FFD) serves to rapidly-detect a breakdown occurring on a power system. When current exceeding a preset value is introduced, the FFD detects the current and thus transmits a signal to a fast switch controller.
The fast switch (FS) is composed of a main circuit contact for applying current and detouring a fault current, and a driving unit. And the fast switch (FS) serves to convert a fault current to a circuit of a current limiting resistor connected thereto in parallel.
The current limiting resistor (CLR) is not provided with current at a normal state, but is provided with a fault current when opening the fast switch (FS) by sensing a breakdown. The current limiting resistor (CLR) is a device for restricting a size of a fault current by its resistance.
FIGS. 1A and 1B illustrates a principle of a fault current limiter. FIG. 1A illustrates a circuit before a fault current limiter is installed, i.e., a circuit where only a circuit breaker is installed. FIG. 1B illustrates a circuit where a fault current limiter and a circuit breaker are installed. When a fault current limiter is installed, a normal current ({circle around (1)}) flows to a load device 102 via a circuit breaker 101 in a normal state. However, when a breakdown occurs, a fault current ({circle around (2)}) flows to the load device 102 by making a detour to a current limiting resistor 105 as a fast switch 104 is open by a fault current limiter 103.
In summary, a fast switch, a component of a fault current limiter, is connected to a current limiting resistor in parallel, so as to effectively control a fault current generated from a power system. The fast switch is a switching device for protecting the power system by rapidly detouring an occurred fault current to the current limiting resistor.
FIG. 2 illustrates a configuration of a fault current limiter in accordance with the conventional art. FIG. 2 illustrates a technique disclosed in Korean Registration Patent No. 10-0955373 (“Hybrid fault current limiter using superconducting device”). The conventional fault current limiter includes a superconducting device 1; a vacuum interrupter 2 connected to a rear end of the superconducting device 1 in series; a rear-end circuit breaker 8 connected to a rear end of the vacuum interrupter 2 in series, and capable of switching a circuit of the power system toward a load side; a permanent magnet actuator 3 for providing a contact force to a movable contactor 2b of the vacuum interrupter 2 when a normal current flows to a power supply line of the power system; a fast switch 5 having a movable contact 5b connected to a mover 3a of the permanent magnet actuator 3 so as to be moveable in a synchronized manner; and a driving coil 4 driven to a closing position for conducting the fast switch 5 by being magnetized by a fault current when the superconducting device 1 is quenched, and driven to an opening position for interrupting the vacuum interrupter 2 by the permanent magnet actuator 3.
An operation of the fault current limiter in accordance with the conventional art will be explained as follows.
In a normal state, current on a circuit flows along a conducting path (A). That is, the current is introduced along a power side line, and passes through the vacuum interrupter 2 via the superconducting device 1 of a non-resistance state, thereby being discharged to a load side through a rear-end circuit breaker 8.
When a short-circuit current occurs on the circuit, resistance of the superconducting device 1 is drastically increased. Thus, the current is smaller than the resistance of the quenched superconducting device 1, and flows along a conducting path (B). That is, the current passes through a current limiting resistor 7 and the rear-end circuit breaker 8, via the driving coil 4 connected to the superconducting device 1 in parallel, thereby flowing to the load side. In this instance, since a repulsive plate 4a is vertically moved by a magnetic force generated at the driving coil 4, the movable contactor 2b and a fixed contactor 2a of the vacuum interrupter 2 are separated from each other. Then, the movable contact 5b and a fixed contact 5a of the fast switch 5 come in contact with each other. Thus, the short-circuit current flowing along the conducting path (B) flows along a conducting path (C) connected to the load side via the closed fast switch, the current limiting resistor 7 and the rear-end circuit breaker 8.
However, in the conventional art, a fault current (short-circuit current) is used when an opening operation is performed. This may cause an operation speed to be variable according to a size of the fault current. And, there may exist a proper fault current section for completion of the opening operation. That is, when a fault current is small, an electronic repulsive force is small. This may cause the fault current limiter not to operate. On the other hand, when a fault current is too large, the circuit is immediately re-closed by a mechanical repulsive force.